1. Field of the Invention
This invention relates to operations on underground pipelines, and more particularly to apparatus for pulling a traction cable, of the type having a frame supporting means for pulling the cable.
2. Description of Related Art
In view of the trouble and expense occasioned by civil engineering work extending along an excavation when it is desired to carry out an operation on an existing pipeline, especially in order to replace it, various means have been proposed, e.g., operations solely in the vicinity of the manholes of the pipeline, whereby both the expense and the disadvantages of such construction sites may be substantially reduced.
In general, the operation takes place between two manholes disposed at ground level at each end of the pipeline on which the operation is to be carried out; a first manhole, the entry manhole, is intended to introduce the objects or tools necessary for the operation, while the second manhole, the exit manhole, is generally provided with a winch or other traction means responsible for pulling the object or tool along the pipeline by means of a cable. According to the type of operation, particularly when the tool is an air hammer intended to break up the pipe to be replaced, the tractive force may be considerable, up to 200 kN or more.
For such a pull, particular precautions must be taken in connection with the winch, especially as regards its stability and the way it is secured over the exit manhole or at the end of the pipeline. One solution is to make a large excavation in the vicinity of the exit manhole in which to place the winch, usually mounted on a trailer, so as to position the entry axis of the cable on the drum opposite the end of the pipe. This solution is relatively costly because of the expense of the necessary excavation.
Then, the winch has been kept on the surface and the necessary return pulleys installed, by means of independent pulleys, so as to guide the traction cable from the end of the pipe to the drum. Though this solution may be advantageous when the tractive force involved is low, it is not at all suitable in the case under consideration here in view of the extensive securing facilities required, both for the winch and the pulleys.
An improvement on the preceding process has been to use a strong metal beam to connect the winch frame to the return pulley disposed directly opposite the end of the pipe in order to take up the vertical stress between these two elements. Until now, the proposed solutions of this type have included connecting the beam to the winch frame by detachable or positionable fixing means, which has limited the amount of force which could be transmitted from the beam to the frame.
Since the axis of the pipe is very often shifted relative to the vertical axis of the chamber, i.e., relative to the center of the manhole, the beam must be disposed at an slant in order to compensate for this shifting. Thus, a weak attachment between the beam and the frame cannot absorb the torsional stress applied to it.